Acoustic apparatus



Oct. 10, 1967 J. v. BoUYoucos 3,346,058

ACOUSTIC APPARATUS Filed May 29, 1964 3 Sheets-Sheet 1 limo m LMuo RETURNCHANNEL I .IA. II:

MUD MODULATION ELEMENT #3 & o I 44 5 i 3 E 31:! o I INVENTOR. kpac +1 .E F F R E i T JOH/V v. aouroucas AH- f BY Kala h ATTORNEY Oct. 10, 1967 J. v. BOUYOUCOS ACOUSTIC APPARATUS I 5 Sheets-Sheet 2 Filed May 29, 1964 /IIO ATTORNEY I Ii I 1 EFFECTIVE TERMINATION Oct. 10, 1967 Filed ma 29. 1964 J. v. BOUYOUCOS ACOUSTIC APPARATUS 3 Sheets-Sheet 5 JOH/V V. BOUYOUCOS ATTORNEY BY @mmhfilgw wk I United States Patent 3,346,058 ACOUSTIC APPARATUS John V. Bouyoucos, Rochester, N.Y., assignor to General Dynamics Corporation, a corporation of Delaware Filed May 29, 1964, Ser. No. 371,202

, .11 Claims. (Cl. 175-56) This invention relates to acoustic apparatus, and more particularly to methods of and apparatus for drilling or boring holes in the earth with the aid of acoustic apparatus of the type adapted to operate with drilling fluids.

The invention is especially suitable for use in oil well drilling and other apparatus where deep boreholes are drilled into the earth with the assistance of drilling fluids, such as are known in the oil well drilling art and referred to as drilling muds.

At the bottom of deep holes the pressure due to the weight of the drilling mud in the borehole may exceed the pressure exerted by the earth formation on the bottom of the hole. The interface between the formation and the bottom of the hole becomes significant. The excess pressure exerted on the bottom hole formation interface may prevent or retard the removal of chips and other debris which are excavated by the drill bit. On the other hand, the pressure on the side-hole formation interface may be desirable to support the side-hole against collapse. Especially significant is the effect of the bottom hole pressure in holding down the chips broken off the earth formation by the drill bit. If the chips are .not removed, immediately after they have been formed, the drill bit will continue to waste energy in' crushing the chips thereby leading to increased drill bit Wear and reduction of drilling rate. Worn bits need replacement. Frequent bit replacement appreciably increases drilling time.

It is an object of the present invention to provide methods and apparatus for drilling holes which afford faster drilling than is the case with known drilling methods and apparatus.

It is another object of the present invention to provide improved tools for forming boreholes in the earth especially at great depths.

3,346,058 Patented Oct. 10, 1967 vibrations are established, utilizing the energy of the circulating fluid, to. modulate the pressure of the fluid in the bottom hole region. Periodic variations in bottom hole pressure are generated which oppose periodically the hold-down forces which tend to prevent the removal of chips or other debris. The drilling rate is increased since the adverse effect of these chip hold-down forces are advantages thereof will become more readily apparent It is a further object of the present invention to provide improved methods and apparatus for earth drilling and boring which improves the drilling process by facilitating the removal of material, such as chips and the like from the bottom of boreholes. 1

It is a still further object of the present invention to provide methods of and apparatus for earth boring and drilling which minimize the development and retention of caked drilling mud and other debris between the drill bit and the formation which can hinder the drilling process. p Y v It is a still further object of the present invention to provide improved apparatus for modulating the flow of drilling fluids so as to assist in drilling.

It is a still further object of the present invention to provide improved acoustic vibration generating apparatus, especially suitable for use in drilling and boring tools.

It is a still further object of the present invention to from a reading of the following description in connection with the accompanying drawings in which:

FIG. 1 is a diagrammatic view of an oil well drilling system which incorporates the present invention;

FIG. 2 is a graph illustration the improvement in drilling rate obtained through the use of the invention;

FIG. 3 is a diagrammatic sectional view of a drill tool incorporating the present invention, in accordance with one embodiment thereof, in section beingtaken along a trarlisverse plane through the longitudinal axis of the too I FIG. 4 is a schematic diagram of the acoustic circuit of the apparatus shown in FIG. 3;

FIG. 5 is a fragmentary sectional view of a drilling tool in accordance with another embodiment of the present invention, the section being taken along a transverse plane through the longitudinal axis of the tool;

FIG. 6 is a graph showing the pressure distribution along the axis of the borehole drilled by a tool embodying the invention;

FIG. 7 is a fragmentary, diagrammatic sectional view of a drilling tool, similar to the tool shown'in FIG. 3, which incorporates means for providing increased protection against-damage of the side walls of the borehole;

, FIG. 8 is a bottom view of the tool shown in FIG. 7;

'FIG. 9 is an acoustic circuit of a portion of the apparatus shown in FIG. 7;

FIG. 10 is a sectional view of a drilling tool in accordance with still another embodiment of the present inventionythe section being-taken along a transverse plane provide improved acoustic vibration generating apparatus which operates with drilling fluids, such as muds.

It is a still further object of the present invention to provide improved apparatus for forming boreholes in earth formations which will'not damage the side walls of the hole.

Briefly described, the invention may be included with a tool which is adapted to drill a hole in earth formations by the generation and removal of chips and other debris from the bottom of the hole. The drilling-fluid is circulated into and out of the bottom hole region. Acoustic through the axis of the tool; and

FIG. 11 is a schematic diagram of the acoustic circuit of the tool shown in FIG. 10. I 'Referring to FIG. 1, a derrick 10 is situated over the hole 12 which is bored in the earth in an effort to find oil, a gas, or similar deposits. A hoist 14, such as a block and tackle, is attached to the top of a drilling apparatus 13 for the purpose of raising and lowering the apparatus in and out of the hole. The apparatus 13 itself includes 1 a drill string 16, of the type which is used in the oil well drilling art, such as a hollow pipe or plurality of pipe sections coupled to each other. The bottom of the drill string is connected to a drill collar 18. Following one or more of these collars, or between successive collars, is a drilling mud modulation element 20. The. element 20 modulates the mud by generating acoustic vibrations therein so as to periodically vary the pressure of the mud in the bottom hole region, as will be explained more fully hereinafter. A drilling head 22, which may be of the type adapted to be rotated, is attached to the modulation element 20 or to the last of the collars 18. By drilling head is meant bits, housing, flow couplings, mechanical attachments and other parts constituting an assembly, the purpose of which is excavation at the bottom of the hole. A mud inlet channel is provided by communicating passages through the drill string 16, the collar 18, the element 20 and the head 22. The mud passes through the region adjacent to the head 22 at the bottom of the hole. The mud returns to the surface through a channel between the outer periphery of the drill string 16, the collars 18 and the element 20 and the periphery of the hole 12.

Mud is pumped into the inlet channel by means of a mud pump 24 which may receive mud from a suitable sump or reservoir and pump this mud into an inlet 26 at the top of the drill string 16. Mud is returned from the mud return channel into the mud pump or sump for recirculation by the pump to the drill string, after suitable filtration to remove debris carried up by the mud.

A rotary table 28 coupled to the drill string 16 and driven by a drive motor 30 rotates the drilling apparatus. As the drilling head 22 rotates, material is excavated from the formation at the bottom of the hole. This material may be in the form of the chips. The action of the drilling mud normally is to carry away the removed material by washing or flushing.

As the drill bit penetrates deeper and deeper into the earth formation, the hydrostatic pressure at the bottom of the hole principally due to the weight of the vertical column of drilling mud continually increases. When the internal pressure (P in the drilling mud at the bottom of the hole exceeds the subsurface; pressure (P exerted through the earth formation on the bottom of the hole, the removal of chips excavated by the drilling head is retarded. The positive pressure differential, P -P exerted across the bottom hole formation interface acts hydraulically to hold down chips that are formed, instead of allowing them to be flushed away by the action of the drilling mud. Also, the pressure differential can cause a mud cake to be formed between the bit and the bottom hole region, particularly in the case of porous formations. Accordingly, the drilling rate will decrease with an increase in the positive differential pressure, P P across the bottom hole formation interface. The bit is retarded because it first crushes chips already formed before reaching the virgin bottom hole formation.

The decrease in drilling rate with increasing pressure differential is illustrated in the graph shown in FIG. 2. The drilling rate decreases monotonically with increasing pressure differential, the decrease being rapid at first up to a point after which the decrease in drilling rate with increasing pressure diflerential tends to level off asymptotically to a low substantially constant rate as the pressure dilferential is further increased.

The mud modulating element utilizes the energy in the pressurized mud flowing through the mud inlet channel to generate acoustic vibrations in the mud. These acoustic vibrations are coupled to the bottom hole region and periodic variation or pulsation in internal pressure (P results. This variation in internal pressure (P,) is indicated by the sine wave 32 in FIG. 2 of peak amplitude P which is superimposed upon a nominal or average value of pressure differential P,,. P,,, in turn, is associated with a nominal or average value of drilling rate q The resultant time variation in the instantaneous value of bottom hole pressure P gives rise to a time variation in the instantaneous value of drilling rate shown as q in FIG. 2. It is to be noted that as Pinst swings toward a low value of pressure differential, P q swings toward a high value of drilling rate, q As P swings toward a high value of pressure differential, P q swings toward a low value of drilling rate q Since the characteristic drilling rate vs. pressure differential curve is cating a net increase in drilling rate as a result of the applied time variation in pressure differential. Accordingly, the modulation of the mud pressure has effectively increased the drilling rate from the value q to :1

The applied pressure modulation as shown in FIG. 2, is not sufiicient to cause the instantaneous pressure differential P -P to go negative over any portion of the cycle. If however, P is made large enough for P,P to become negative over a significant portion of a halfcycle, the resultant reversal in bottom hole pressure differential can actually force chips away from the bottom of the hole, thereby increasing substantially the drilling rate.

In FIG. 3 one embodiment of an oil well drilling tool incorporating the invention is illustrated. A cylindrical drill collar 34 is shown connected to the upper end of a mud modulating element housing 36 which may be cylindrical in shape. The collar has a hole 37 therethrough which provides the mud inlet in the collar 34. The mud is not shown in place in the apparatus to simplify the illustration. M uds or other hydraulic drilling fluids of the type known in the art may be used. Screw connections between the collar 34 and the housing 36 are shown. It will be appreciated that bolt and flange or other connecting not a straight line, it can be seen that the peak increase 2 in drilling rate, q q occurring during the first portion of the cycle shown, is considerably greater than peak decrease -in drilling rate q q occurring during the second portion of the cycle shown in FIG. 2. As a result, the average drilling rate, which is the time average of q in FIG. 2, has moved from the value q to q indimeans may be provided if desired between the collar 34 and the housing 36 or between the collars and drill string sections, if desired.

The housing 36 is interposed between the collar 34 and a drilling head 38. The head 38 includes a shaped housing 39 having a cupped end face 41 to which three drill bits 40 of the conical type are attached one hundred twenty degrees apart from each other. The head 38 also has a passage hole 42 therethrough through which the mud may flow into a region 44 adjacent at the bottom of a borehole 46.

The housing 36 includes three successive cylindrical sections 48, 58, and 90 which are screw threaded for attachment one to another. The section 48 has a passage therethrough formed by an upper hole 50, a cavity 52, a narrow bore 54, which may be of smaller diameter than the hole 50, and a chamber 56, all along the axis of the housing 36 and coaxial therewith. The upper hole 50 is circular in cross-section and communicates with the mud channel hole 37 in the collar 34. The cavity 52 and the chamber 56 both have cylindrical side walls, the upper and lower ends of which may be conical in shape to reduce turbulence in the flow path.

The intermediate section 58 is an assembly of parts including cylinder 59 which has a hole 62 of circular cross-section coaxial with the housing 36 bored into its upper face. The bottom of this hole 62 defines a shoulder 64. A cup shaped member 66 is inserted into the hole 62 and rests on the shoulder 64. The bottom of this cup shaped member 66 has a circular hole 68 concentric with the axis thereof and also concentric with the axis of the housing 36. The member 66 provides a stator port for a valving mechanism and the hole 68 through the member 66, with a valve member 82 to be described more fully hereinafter, define a valve orifice. Thus, the members 66 and 82 are desirably formed from tungsten carbide or other hardened material to improve their wear characteristics.

Three webs 70, two of which are shown in FIG. 3, extend from the walls of a passage hole 72 through the bottom face of cylinder 59. These webs support an elongated tube 74 which is closed at its bottom end 76. An L-shaped channel 78 in the cylinder 59 and a communicating L-shaped channel 80 in the lower portion of the upper cylinder 48 communicate the interior of the tube 74 and the interior of the cavity 56 with each other, for valve centering purposes, as will be explained more fully hereinafter.

A valve 82 is located in the intermediate assembly 58 and is a cylindrical body having a relatively thin disc-like portion 84 and a relatively thick disc-like portion 86 respectively at the upper and lower ends thereof. These disc portions are separated by ribs 88. The thin disc portion 84 is adapted to move through the hole 68. Theupper circular edge '65 of the stator member 66 and the lower circular edge 67 of the disc portion 84 control and throttle the flow of mud and cooperatively define the valve orifice referred to henceforth by the same reference numeral (68) as the stator hole 68. The valve 82 oscillates in a self-excited mode in response to the passage or circulation of the drilling mud and establishes acoustic-vibrations therein, as will be explained in greater detail hereinafter.

The cylinder 59 is attached to a cylinder 90 constituting the lower-most of the sections. The cylinder 90 is axially bored with upper and lower circular holes 92 and 94, respectively of larger and smaller diameter and coaxial with the housing 36. The lower hole 94 joins the mud channel hole 42 in the drilling head 38. The tube 74 is dispose-d partially within and coaxial with the upper hole 92. When the drilling head 38 is attached to the bottom of the cylinder 90, by means of the threads in their mating faces, the holes 94 and 42 communicate with each other. A passage for the circulation of drilling mud therefore extends along the axis of the drill collar 34, the housing 36 and the drilling head 38. This mud fiows, acoustically modu-v late-d, into the bottom region 44 of the borehole 46. The mud is returned to the surface in the channel between the inner periphery of the hole 46 and the outer periphery of the head 38, housing 36, collar 34, and drill string sections. The inlet and return flow paths are, respectively, illustrated by two arrows 96 and 98.

When the pressurized drilling mud flows into housing 38 through the upper hole 50, it fills the cavity 52. The cavity 52 acts like a large acoustical compliance effectively isolating the portion of the drilling apparatus including the collar 34 and the rest of the drill string [from the valvin-g mechanism 66-82. Accordingly, strong acoustic vibrations generated in the modulating element 36 do not pass upwardly beyond the cavity 52.

The drilling mud circulates through the cavity 52 into the narrow bore 54. The bore 54 is dimensioned so that, when filled with drilling mud, it acts as an acoustic mass or inertance. The chamber 56 fills with mud. The filled chamber presents a relatively large stiffness reactance as compared to the inertive reactance of the bore 54- and may be neglected in the acoustic circuit of the apparatus. The mud also fills the space inside of the cup shaped member 66 and also fills holes 72 and 92. The mud also fills the tube 74. The valve 82 is centered radially in the tube 74 by its thick disc portion 86. The disc portion 86 has a diameter slightly less than the inner diameter of tube 74 to provide a clearance for axial motion of -the valve 82.

This clearance is sufficiently small, however, to minimize 6 r erence may be had to I. V. Bouyoucos application Ser. No. 192,274, filed May 3, 1962, now Patent No. 3,143,-

999, for a further description of static valve location.

The body of drilling mud in the tube 74 is relatively large and therefore acts as an acoustic stiffness element. In other Words, the mud in the tube is compressible and behaves as a liquid spring which is extended and compressed in response to the pressure on the valve 82 due to the circulating mud. The valve 82 oscillates in a sel f-sustaining manner, as may be understood from the acoustic circuit of the apparatus.

The simplified, equivalent acoustic circuit of the apparatus shown in FIG. 3 is illustrated in FIG. 4. A battery 100 represents a source of hydraulic pressure differential between the entrance to the cavity 52 and the bottom hole region 44, and is due to the circulating mud. The acoustic compliance C presented by the cavity 52 is effectively across the pressure source 100. An acoustic inertance M presented by the bore 54 is in series with the acoustical inertance M presented by the valve. The mud in the tube 74 is a stiffness element presenting a compliance C in series with the inertance of the valve 82. Effectively in shunt with this circuit including the inertance M and the compliance C is an acoustic resistance R presented by the valve orifice 68. In series with the valve orifice are successive acoustic inertances M M and M presented, respectively, by the mud in the holes 92, 94, and 42. The mud in the bottom hole region 44 and the mud in the return channel also presentsinertances M and M which are in series with R M M and M The acoustic circuit provided in the housing 36 exhibits reso nance at a certain frequency depending primarily upon the values of M M and C However, the other elements, eg. M M M M and M also are included in the circuit and also may have a slight effect on the determination of the resonant frequency of the circuit.

The resonant frequency is primarily determined by a tank circuit defined by the circuit branch including M and C in parallel with the circuit branch including M and C At the desired resonant frequency of the tank circuit, the reactance of C is greater than the reactance of M and the reactance of M is greater than the re actance of C The reactance looking into the branch including M is thus primarily a stiffness reactance, while the branch including M presents primarily in inertive reactance. The circulating mud is effectively introduced acoustic losses in the annular gap between the cylindrical piston member defined by the thick disc portion 86 and tube 74. The quiescent or static position of the valve, by which is meant the average position of the valve, considerin-g that it normally oscillates reciprocatively along its longitudinal axis, is dictated by the static pressures in the chamber 56 and tube 74. The static pressures in the cavity 56 and in the tube 74 are equalized, since the chamber 56 and the inside of the tube 74 communicate with each other through the channels 78 and 80. The channels 78 and 80 are preferably small in cross sectional area to present a high acoustic impedance compared to the acoustic impedance seen looking into cavities defined "by the chamber 56 and tube 74. Thus, aside from equalizing the static pressures in the chamber 56 and tube 74, these channels 78 and '80 do not affect the dynamic acoustic circuit to be described hereinafter. Forces are exerted onthe valve 82 due to the pressures in the chamber 56 .andtube 74 and also due to the momentum of the mud which flows through the valve orifice 68. The areas of the upper or lower faces of the valve 82 may be slightly different so that the static forces tend to locate its thin disc portion immediately above the top of the stator member 66. Refat the junction of the two branches and tends to sustain the oscillations of the valve 82 at the resonant frequency determined by the circuit so that the valve oscillation frequency and the circuit resonant frequency are about equal to each other. This frequency is desirably in the acoustic range, say 200 cycles per second. The flow through the valve orifice 68 is periodically varied in accordance with the valve oscillations. Acoustic pressure vibrations are thereby established in the circulating mud, which pressure vibrations are effective in the bottom hole region 44. Reference may be had to Patent No. 3,004,512, issued to J. V. Buoyoucos et al. on Oct. 17, 1961, for more general description of acoustic vibration generators.

It may be desirable to construct the housing 36 so that the length of the mud passage from the valve orifice 68 to the bottom hole region is approximately a quarter Wavelength at the frequency of oscillation of the valve 82. Then a standing wave of pressure is established, which wave may have a maximum point at the bottom hole re gion 44.

It may be desirable in order to prevent appreciable transmission of vibratory energy along the mud return channel adjacent to side walls of the hole 36 to provide an acoustic termination approximately one-quarter wavelength at the oscillation frequency from the bottom of the hole. Such an acoustic termination may be any compliant member which retains its compliance in spite of the high hydrostatic pressures found in its vicinity. A suitable termination may, for example, be a hollow cylinder having a flexible outer wall and being filled with pressurized gas, such as nitrogen (see FIG. 10). Other forms of acoustic termination known in the art may be used.

A portion of another mud modulating element is shown in FIG. 5. This element is adapted to be connected between a drill collar and a drill bit assembly similarly with the mud modulating element shown in FIG. 3. Parts of the element shown in FIG. which are similar to parts of the element shown in FIG. 3 are designated by light reference numerals with primes appended thereto. A massive, cylindrical valve member 102 is statically centered with its upper face immediately below the hole defining the valve orifice 68. The mass of the valve 102 is sufficiently great so that its inertive reactance at the frequency of resonance of the system is greater than the stiffness reactance of the mud in the tube 72'. Accordingly, the valve 102 acts as an inertance or mass controlled element. While oscillating, the displacement of the valve is approximately one-hundred-eighty degrees out of phase with the force applied thereto. The top end face of the valve 102 is statically located below, rather than above, the lip of the hole in the stator member which, with the valve 102, defines the valve orifice 68'. The static location of the valve 102 may be accomplished by providing additional narrow channels through the body of the valve between its lower end face and its outer periphery. The pressure differentials across these channels tend to locate the valve by reference to a fixed point such as the upper rim of the tube 72'. In all other respects, the operation of the mud modulator shown in FIG. 5 is similar to the operation of the mud modulator shown in FIG. 3.

FIG. 6 graphically represents the standing wave of pressure which may be generated in the system. Assuming the system is terminated approximately one-quarter wavelength from the bottom hole region 44 by means of a relatively soft compliant structure, a node and a maximum point of the standing wave are established at the termination and at the bottom hole region 44, respectively. The acoustic path between the modulating element, the bottom hole region 44 and the termination includes both lumped and distributed acoustical elements. Accordingly, the physical length of the path may not equal exactly one half wavelength. The valve orifice of the modulating element is desirably effectively one-quarter wavelength from the bottom hole region. The effective wavelengths are measured at the frequency of the generated acoustic vibrations in the fluid.

Referring to FIGS. 7 and 8, there is shown a drilling head 104 connected to a mud modulating element 106, similar to the mud modulating element shown in either FIG. 3 or FIG. 5. The drilling head 104 includes a cylindrical sleeve 108 which extends upwardly from the head, over the region where the acoustic-vibration pressures are largest (say about one-quarter wavelength at the frequency of the generated acoustic vibrations). The sleeve 108 and the outer peripheries of the head 104 and of the mud modulating element housing 106 define a circular mud flow return channel 110 therebetween.

The sleeve 108 may be attached to the drilling head 104- by bolts or other suitable fastener means, or may be integral with the head as shown. The sleeve thus has the same diameter or gage as the borehole. Orifices 114 are drilled into the bottom face 116 of the drilling head 104. These orifices have ports at the annular channel 112 defined in the head 104 at the bottom of the mud channel 110. Other ports of the orifices 114 at the bottom face 116 are between the bits 118 (see FIG. 8).

Mud circulates through the passage 120 in the modulation element and in the head 104 into the bottom hole region adjacent to the bits 118. This mud is acousticvibration modulated to assist in the drilling process. Modulated mud containing the chips and other debris enters the channel 110 through the orifices 114. The outer periphery of the sleeve 108 is close to wall of the borehole. The mud in the channel 110 varies in pressure in 8 accordance with the modulation thereof in the modulating element 106. Since the sleeve 108 is interposed between the modulated flow of mud and the walls of the borehole, the sleeve 108 tends to reduce any damage to the walls of the hole due to the pressure variations in the mud which flows through the channel 110. These pressure variations are substantially reduced in amplitudeat the upper end of the channel 110. Accordingly, the mud after passing through the channel 110 is not likely to do significant damage to the borehole walls. Depending upon the strength of the formation through which the borehole is drilled, the sleeve 108 may or may not be used. It is desirable for weak formations.

FIG. 9 shows a simplified acoustic circuit of the apparatus shown in FIGS. 7 and 8. The source 122 represents the oscillating pressure of the mud established by the mud modulating element at its output. M represents the inertance of the passage 120 and the passages through the modulating element adjacent thereto (e.g. FIG. 3, passages 94, 92, and 70). C represents the acoustic compliance of the fluid in the bottom hole region. The compliance of the bottom hole region may dictate the acoustic characteristics of the circuit, since the volume of the fluid in the bottom hole region may be large as com pared to the volume of the fluid in the channels 120 and the channel 110. M represents the inertance of the fluid in the orifices 114 and channels 110 and 112. R represents a resistive load provided by the fluid return channel above the sleeve 108. Since the bottom hole cavity may be effectively large and thereby presents a stiffness reactance rather than an inertive reactance in the acoustic circuit, the acoustic pressure may be maximized in the bottom hole region. Also, a little acoustic energy is transmitted into the fluid return channel by suitably p-roportioning the channel 110 and passage 120 to effectively provide a low pass filter having a cutoff frequency below the frequency of vibration of the mud.

Referring to FIG. 10, another drilling tool is shown having a mud modulating element 124 and a drilling head assembly 126. The mud modulating element itself includes a cylindrical housing connected at its upper end to a drill collar 128. An upper hole 132, an intermediate hole 133 and a lower hole 134 are bored through the housing 130. All the holes may be circular and coaxial. The upper hole 132 provides an inlet channel for the circulating mud. The intermediate hole is much larger in diameter than the upper hole and communicates therewith through a conical section 136. The hole 133 defines a cavity whiqh isolates the upper portion of the mud circulating channel acoustically from the modulating element. A plurality of webs 138, two of which are shown in FIG. 10 support an elongated tube closed at its upper end 142. A cylindrical valve 144 is located in the hole 133 with its upper portion centered within the tube 140. The bottom of the hole 133 defines a shoulder 146. The rim of this shoulder and the portion of the wall of the lower hole 134 which extends immediately below the shoulder 146, with the circular edge of the end face of the valve 144, define a valve orifice 148. A plurality (for example, three) L-shaped feet tend to center the lower portion of the valve coaxially within the hole 134.

A pinion gear 152 is disposed within the hole 134 below the valve orifice 148 and journalled to the housing 130. A rack gear 154 engages the pinion gear 152. The upper end of the rack gear is attached by means of a spring 156 to the bottom of the valve 144. Aside from mechanically coupling the rack gear 154 to the valve 144, the spring also may aid in locating the valve with respect to the valve orifice defining shoulder 146 and as a stop to limit the travel of the valve 144. Other hy draulic valve locating circuits are mentioned hereinafter.

The drilling head assembly 126 includes a neck portion 158 which extends into the lower hole 134. The lower hole 134 is cut away to receive the neck 158 of the bit assembly 126. A shoulder 160 of the cut away 9 portion limits the axial movement of the assembly 126 in the upward direction. A rack gear 162 extends from the upper end of the neck 158 and engages the pinion 152. Cooperating splines 164, 166 in the inner periphery of the cylindrical housing 130 and in the outer periphery of the neck 158 permit non-rotative translation of the housing 130 and neck 158.

The assembly 126 also includes an end portion 168 to which three rotatable, conical bits 1'70 are journalled. The neck 158 and end portion 168 may be integral, if desired. A flanged end 172 of the neck portion is interposed between the end portion 168 and the bottom of the housing 130. A plurality of channels 174, two of which are shown, extend through the housing 130, the flanged end 172 of the neck portion 158 and the end portion 168 in a direction along the common longitudinal axis thereof. The channels terminate at the end face of the end portion between the drilling bits 170 (FIG. 8) so that they communicate the bottom hole region with the intermediate hole 133 in the housing 130.

In operation the drilling mud flows through the upper hole 132 and fills the intermediate hole 133 which functions as a cavity having relatively large compliance when filled with the drilling mud. The tube 140 also fills with the drilling mud. A liquid spring is provided by the drilling mud contained in the tube 140. The mud flows through the valve orifice 148 and fills the lower hole 134. The drilling mud circulates through axial holes 176 and 178, respectively, in the neck and end portions 158 and 168 of the drill assembly 126. The mud then flows into the bottom hole region (similar to region 44, FIG. 3). The return channel for the mud is provided between the wall of the borehole and the outer periphery of the drill assembly 126, mud modulating element 124 and the remaining drill collars and portions of the drill string ;(see FIG. 1). Mud passed through the channels 174 is expelled in jets against the bottom of the borehole to assist in removalof chips and the like.

The simplified equivalent acoustic circuit of the mud modulating element 124 is shown in FIG. 11. A battery 180 represents the hydraulic pressure differential between the entrance to the cavity presented by the hole' 133 and the bottom hole region. C represents the acoustic compliance of the fluid in that hole 133 and is effectively across the pressure source 180. Since the mud flows through the valve orifice 148, the resistance R of that orifice is in series with a branch circuit 191 including an inertance M which represents the mass of the valve 144 and an acoustic compliance C presented by the fluid in the tube 140. Another branch circuit 193 in series with the valve orifice resistance R includes a number of inertance elements due to the mud in the successive holes and passages indicated as M M M M and M The mud in the holes 134, 176, and 178 and in the bottom hole and return channel regions may effectively be inertance elements looking toward the bottom hole from the valve orifice because of the shape of these holes and the acoustic characteristics of the drilling mud.

The two branch circuits 191 and 193 define a parallel resonant circuit which is resonant at a frequency where the branch 191 presents an effective stifiness reactance which is equal to the effective inertive reactance in the other branch 193. This resonant frequency is suitably arranged to be in the acoustic range, say two hundred c.p.s.

by configuring the holes in the mud modulating element, the mass of the valve, and the acoustic compliance of tube 140.

The oscillation frequency of the valve is approximately equal to the resonant frequency of the circuit. The valve periodically derives energy from the circulating mud which flows through the valve orifice 148 as the valve oscillates, so that the valve oscillation is sustained at the oscillation frequency.

The mud channel including the holes 134, 176, and 17 8 as well as the bottom hole region, are all in the resopressure accordingly varies at the oscillation frequency in the mud channel and in the bottom hole, region. The

1 pressure variations are larger in the bottom hole region since that region presents an acoustic compliance, C

(FIG. 11). M M and C define a pressure maximizing circuit that enables the acoustic pressure-s in the branch 193 to be high in the bottom hole region. The elements in the branch 193 nevertheless present an inertive reactance which resonate with the stilfness reactance of the branch 191 including the valve inertance.

An acoustic termination is provided which may be in the form of an annular groove 182 in the housing 130. The groove is sealed by a resilient ring 184 and is filled with a gas, such as nitrogen. This termination may effectively return M to the point of acoustic reference pressure.

In order to prevent any chips or aggregate material from clogging the valve orifice 148, the drill string may be lifted slightly. The bit assembly 126 being in engagement with the bottom hole region tends to remain in engagement therewith. Thus, the cylindrical housing 130 moves upwardly while the neck portion 158 of the bit assembly 126 remains stationary. This relative movement of the housing 130 with respect to the neck portion 158 causes counterclockwise rotation of the pinion gear 152. The counterclockwise rotation of the pinion 152 raises the valve 144 to the position indicated by the dashed lines 145 and clears the valve orifice 148 so that debris can flow through the orifice. When the orifice is cleared, the mud modulating element is dropped back in place and drilling may then resume.

A narrow U-shaped channel which presents only a resistive acoustic impedance connects the tube 140 and the lower hole 134 in the housing 130 and equalizes the static pressure therein. The hydrostatic pressure of the mud in the lower hole 134 and in the tube 140 is balanced so as to aid in locating the valve 144 so that its average position is with its bottom end face just above the valve orifice 148. The relative areas of the end faces of the valve 144 may be different or channels through the valve may be provided also to aid in locating the'valve 144 statically, as was mentioned above.

From the foregoing description, it will be apparent that there has been provided improved methods and apparatus for earth boring and drilling which are especially adapted for drilling deep boreholes in earth formations for sinking oil and gas wells. While several embodiments of the invention have been described, various modifications in the method and in the elements of the apparatus will, no doubt, become apparent to those skilled in the art. Accordingly, the foregoing description should be taken as illustrative and not in any limiting sense.

What is claimed is:

1. Apparatus for drilling a borehole in the earth comprising (a) a drill string adapted to be lowered into. the hole and having a channel therethrough,

(b) a drill bit for excavating the bottom of said bore hole,

(c) a mud flow modulating element for generating periodic pressure pulses including a fluid actuated valve means adapted to generate acoustic vibrations in fluid passing therethrough, said mud flow modulating element being disposed between said bit and said drill string,

(d) means for circulating a drilling fluid through said drill string channel, said modulating element, and said bit for discharge into said hole bottom, said mud returning to the surface of the earth along a path adjacent the walls of said borehole, said element establishing periodic variations in the fluid pressure at said hole bottom in response to the fluid vibrations generated in said element, and

(e) isolation means for-med in said drill string and 11 acting upon said return path for confining the pressure modulations within said bore hole adjacent to said bit.

2. Apparatus for drilling boreholes in earth formations with the aid of a drilling head engageable with the bottom of said borehole into which drilling mud is adapted to be introduced, said apparatus comprising (a) a housing having a passage along the longitudinal axis thereof for the flow of said mud therethrough,

(b) said passage being contoured to define regions of different cross sectional areas and having at least a first and a second region therein respectively disposed successively in the direction of flow of the drilling mud, said first region having a relatively large volume and defining a stiffness reactance at a certain frequency of acoustic-vibration in said mud, and said second cavity defining an inertance at said certain frequency,

() means providing a valve orifice on said housing and disposed in said passage in one of said regions,-

(d) a free floating valve member disposed in said housing and being reciprocatively movable along a path into and out of said orifice for modulating the flow of said mud, said valve presenting an inertive reactance at said frequency,

(e) means disposed within one of said regions for providing a cavity into which at least a portion of said valve member is movable, said cavity presenting a stiffness reactance at said frequency which with the inertive reactance of said valve member, the inertive reactance of said second region, and the acoustic reactances of the regions communicating therewith defining an acoustic circuit resonant at said certain frequency,

(f) means in said housing for coupling said drilling head to said housing and communicating said housing passage and said borehole bottom, and

(g) isolation means formed in said housing and acting upon said return channel for confining the pressure modulations within said bore hole region adjacent to said bit.

3. The invention as set forth in claim 2 including means providing an acoustic termination in said housing, said termination being diposed about one-quarter wavelength from said borehole bottom and one-quarter wavelength from said valve orifice, the wavelength being defined at said certain frequency.

4. The invention as set forth in claim 2 including means coupled to said valve member and said bit head for moving said valve member clear of orifice when said housing is translated in a direction away from said borehole bottom.

5. The invention as set forth in claim 2 including a plurality of channels extending longitudinally through said housing and communicating with said bit head for supply jets of said mud to said bottom of said borehole.

6. The invention as set forth in claim 2 including a sleeve around said housing and the outer periphery of said drilling head, said sleeve being spaced from said housing and head outer periphery to define an annular channel therewith, said drilling head having orifices communicating the bottom of said borehole and said channel.

7. Apparatus for drilling a bore hole in an earth formation with the aid of a source f p riz fl i n a drill bit disposed in a bit cavity at the bottom of said hole, said apparatus comprising '(a) an acoustic vibration generator coupled to said source for delivering fluid to and substantially modulating the pressure of the fluid at said bit cavity, said generator including (i) a housing having passage means for conducting the flow of fluid from said source through said generator to said bit cavity and from said bit cavity through a return channel formed between said housing and said earth formation,

(ii) a valve mechanism supported in said housing and including a valve member disposed adjacent a stator port and adapted to move in relation to said stator port between an open first position and a fluid flow throttling second position,

(b) said passage means comprising a transmission line between said bit cavity and said stator port and communicating said stator port with said bit cavity for modulating the pressure in said bit cavity,

(c) isolating means in said housing communicating with said return channel for confining the pressure modulation within said bit cavity, and

(d) said passage means, said valve mechanism, and said isolation means being configured to define an acoustic circuit for supporting self-excited oscillatory movement of said valve between said positions.

8. The invention as set forth in claim 7 wherein said housing includes first and second chambers disposed successively in the direction of flow of said fluid, said first chamber having a relatively large volume and defining a stiffness reactance at the frequency'of acoustic-vibration in said fluid produced by said generator, and said second chambers defines an inertance at said frequency.

9. The invention as set forth in claim 8 including liquid spring means disposed in said passage means for acting upon said valve mechanism in said second position urging it toward said first position.

10. The invention a set forth in claim 9 wherein said isolating means is comprised of a compliant member disposed in said housing.

11. The invention as set forth in claim 10 wherein said valve member includes a portion which cooperates with said stator port and a piston portion, said housing defining an elongated tube adapted to be filled with said fluid and upon which said piston portion acts to produce a spring force upon said piston portion aiding the oscillatory movement of said valve.

Refe rences Cited UNITED STATES PATENTS 2,554,005 5/1951 Bodine -55 2,746,721 5/1956 Moore 175-92 X 2,849,214 8/1958 Hall 175-393 X 2,951,682 9/1960 Boucher 175-56 3,018,834 1/1962 Cook 175-92 3,094,176 6/1963 Cook 175-56 X 3,143,999 8/ 1964 Bouyoucos 116-137 3,163,240 12/1964 Bodine 175-56 3,211,243 10/1965 Bodine 175-56 X 3,251,424 5/1966 Brooks 175-56 CHARLES E. OCONNELL, Primary Examiner.

N. C. BYERS, Assistant Examiner, 

7. APPARATUS FOR DRILLING A BORE HOLE IN AN EARTGH FORMATION WITH THE AID OF A SOURCE OF PRESSURIZED FLUID AND A DRILL BIT DISPOSED IN A BIT CAVITY AT THE BOTTOM OF SAID HOLE, SAID APPARATUS COMPRISING (A) AN ACOUSTIC VIBRATION GENERATOR COUPLED TO SAID SOURCE FOR DELIVERING FLUID TO AND SUBSTANTIALLY MODULATING THE PRESSURE OF THE FLUID AT SAID BIT CAVITY, SAID GENERATOR INCLUDING (I) A HOUSING HAVING PASSAGE MEANS FOR CONDUCTING THE FLOW OF FLUID FROM SAID SOURCE THROUGH SAID GENERATOR TO SAID BIT CAVITY AND FROM SAID BIT CAVITY THROUGH A RETURN CHANNEL FORMED BETWEEN SAID HOUSING AND SAID EARTH FORMATION, (II) A VALVE MECHANISM SUPPORTED IN SAID HOUSING AND INCLUDING A VALVE MEMBER DISPOSED ADJACENT A STATOR PORT AND ADAPTED TO MOVE IN RELATION TO SAID STATOR PORT BETWEEN AN OPEN FIRST POSITION AND A FLUID FLOW THROTTLING SECOND POSITION, 